1. Field of the Invention
This invention relates generally to the field of steganography.
2. Description of Prior Art
The art and technology of steganography or secret writing/printing has been practiced from antiquity to the present day. Secret communication methods were widely used in Seventeenth Century England and earlier. In An Annotated Bibliography of Cryptography, David Shulman lists a number of treatises on cryptographic subjects published in England between 1593 and 1776, as well as scholarly books that contained chapters on use of codes, ciphers, and secret writing techniques. One such work, John Wilkins' Mercury, or the Secret and Swift Messenger, describes the use of . . . secret inks and papers. Another source describing the antiquity of various means of secret writing and invisible printing is The use of Encrypted, coded and Secret Communications is an Ancient Liberty Protected by the United States Constitution. By John A. Fraser III., Virginia Journal of Law and Technology, U. of Virginia, Fall 1997 volume 2.
Various ingenious means such as ‘invisible inks’ have been developed to create a hidden mark or message. These hidden marks have been used to conceal messages and to counter the efforts of counterfeiters. In modern times, the art of hidden marks has been extended to the use of bar codes and other information-rich symbols containing variable information. Simple marking has obvious requirements for durability and readability, but in bar-coding and other advanced symbologies, it is desirable to have a means to obscure or hide a mark and make it uncopyable by computer printers.
The prior art consists of embossing and printing bar codes and other information-rich symbols. Bar codes have been used for the identification of documents and products since the early 1950's. Various bar code symbologies have been developed and commercialized, while the technology of scanning and printing has continuously evolved. Numerous developments have been proposed, patented and commercialized, to improve the readability, security and information content of printed codes, including the use of color, encryption, two-dimensional codes, special inks and error correction. Bar-coding is inherently inexpensive to apply because it is printing.
Because it is printing, bar-coding uses ink. All inks are readable through the interaction of the ink with radiation or an electromagnetic field. Visible inks absorb light in the visible spectrum and are thereby readable. Fluorescing or phosphorescing inks are excited by radiation of a particular wavelength and the light emitted is then detectable. Magnetic inks as used in document coding, are detected through their perturbation of a magnetic field. The prior art of printing has made use of various physical properties of inks such as reflectance, absorption, transmission, fluorescence, and color. No printing, especially that of machine-readable symbology has made use of the intrinsic emissivity of materials.
The deficiency in the prior technology is that it is not secure against copying on conventional computer printers and can be read, duplicated, and printed by unauthorized parties. Even fluorescent inks, holograms, and magnetic strips are susceptible to counterfeiting, alteration, and copying.